Mix the tomatoes, onions and tarhana herb in a pot and boil. Lower the heat and simmer until all the vegetables are tender, for about 35 minutes.

Remove the tarhana herb. Mash the vegetables by hand or use a blender.

Place the vegetable purée in a bowl, add the semolina and the flour and mix. Cover the mixture with plastic wrap and leave it to rest for half an hour.

Preheat the oven at 100°C. Place two baking trays lined with aluminum foil. Mix the yogurt with the purée and add as much flour as is needed to make a firm dough that should be quite sticky. 

Place the dough on a dusted surface and knead it until it softens, for about 15 minutes. adding more salt if the dough is too sticky.

Divide it into 10 pieces. Press them with your hands so that they get about 1 cm thick and place them in the baking tray.

Leave them to dry in the oven for about 2-3 hours. (You need less time in ovens with a grill).

Turn the pieces and let them dry for an additional hour or until they become hard enough to crumble.

Blend the pasta in batches. Place the crumbs in the trays for an additional 2-3 hours until they get very hard. (The time required for this procedure is relative). From time to time check the crumbs and stir them.

Leave the trahana to dry at room temperature and then you can store it in airtight jars or cotton pouches (like pillowcases). It keeps for over a year.